Fiber optics can be used for channeling high intensity light over long distances with high efficiency. This is possible due to the high purity and low absorptivity of current fiber optic (FO) materials. The difficulty with transporting high intensity light with FO devices, however, is that the transfer of high intensity light into a bundle of fibers generates a severe heat load that damages the fibers at the interface of the bundle. Intense illumination from a light source often causes the ends of the fibers at the interface to burn, thereby destroying the fibers prematurely.
The heating which results from the transfer of light from a high intensity light source to a FO bundle interface is generally caused by "stray" light rays. Light rays stray from the FO bundle and generate heat in either of two instances. First, if the Numerical Aperture (NA) of the light source is larger than the NA of the fiber optic, then the light rays from the source have a high angle of incidence to their point of contact within the fibers. Rather than being captured by the fibers, these rays are transmitted through the cladding of the fiber into the interstitial spaces between the fibers. These rays are then scattered by multiple reflections, and are eventually converted to heat. The second instance in which light rays stray from the FO bundle relates to the fact that about 10% of the surface area of the face of a hexagonal close packed fiber bundle consists of interstitial voids between the fibers. Light rays which enter these areas directly are not captured by the fiber optics, and are also dissipated as heat.
A promising application for FO technology is the collection of sunlight and the distribution of light therefrom. Fresnel lenses or other known optical devices can be used to concentrate sunlight from a large collection area onto a smaller FO bundle interface. The energy density at the interface can be extremely high. Fresnel lenses generally have a poorly defined NA, though, and some of the focused energy thus escapes the fibers near the interface and generates heat. Even with well designed optics, roughly 10% of the focused energy will always be converted to heat since it directly enters the interstitial spaces present on the FO bundle face.
Glass and quartz fiber has until recently been used for most FO applications. Glass or quartz fibers are very heat resistant. They also have a high heat conductivity. However, the interstitial voids between closely packed fibers act as thermal insulation, preventing the conduction of heat from the interior fibers of the bundle outward. Epoxy has been used to fill the interstitial spaces. However, the temperature of the light which can be focused on the bundle is limited by the temperature sensitivity of the epoxy.
Plastic fiber optic is available which is flexible, rugged and light weight. It also has low optical attenuation. Plastic fiber optic can be obtained at a much lower cost than glass or quartz fiber. Plastic fiber is ideal for many light pipe applications. However, its low softening and melting temperatures (.about.80.degree. C.) make it especially susceptible to damage from excessive heat. Plastics generally have a low heat conductivity. Thus, a bundle of plastic fibers tends to insulate and accumulate heat.
Both plastic and glass FO bundles will be destroyed rapidly when exposed to concentrated sunlight or other focused high intensity light sources, unless some method of cooling the FO bundle interface is used.
U.S. Pat. No. 4,363,080 to Sylvester discloses an apparatus for supplying a source of light by fiber optic for dental applications. A quartz-halogen light is placed in a brass heat sink which contains conduits for the flow of coolant throughout as a means of cooling the lamp. The fiber optic bundle interface is also present in the heat sink and likely benefits from the presence of the cooling conduits. This design utilizes a heat transfer scheme that acts only on the external surfaces of the fiber bundle. Such a heat transfer scheme relies on the bulk thermal conductivity of the bundle material to transfer the internal heat to an outer surface where it can be removed. The poor heat transfer characteristics of FO material severely limit the size and utility of these designs.
A growth market for fiber optic technology is in distribution of lighting within homes and office buildings using fiber optic light pipes. A single solar collector or high intensity source is much more economical and efficient to operate and maintain than a distributed set of smaller collectors or sources. The most expensive component of such a system is the light source. The use of larger or more intense light sources is limited by the lack of an efficient method of cooling the light collection interface.
A known method for cooling the interface of a fiber optic bundle receiving light from a solar collector involves the use of a short length of rod made of quartz or glass fibers which are fused together. The fused fibers of the rod function as light guides between the collection optic which serves as a light source and the fiber light pipes which distribute the light. The rod is jacketed in a finned cylindrical heat sink, with the two ends of the rod exposed. Light from the collection optic is focused on one end of the rod. The other end is placed against the bundle of plastic or glass light pipes. The fused rod collects the focused light with high efficiency since there are no interstitial voids. Rays of light which are outside the NA of the fibers in the rod escape the fiber cladding and are scattered in the rod and dissipated as heat. This heat is conducted along the rod. The sink absorbs the heat before it reaches the joint to the distribution fibers. The light exiting the rod does not contain light rays outside the NA of the light pipes, thereby minimizing the heat produced as the light crosses the butt joint into the distribution fibers.
This method is limited by several factors. The thermal conductivity of the rod is low enough to qualify it as a thermal insulator. The rod diameter thus cannot be made so thick that conduction into the heat sink is slower than the heat production in the rod. Rods with a maximum thickness of 3/8 inch have been reported. In addition, the optical transfer between the rod and distribution bundle is inefficient since fiber to fiber joints are not possible between the fused rod and the unfused distribution fibers.
U.S. Pat. No. 3,611,179 to Flyer discloses a laser device which produces coherent light. Energy generated from the heated cathode is reflected into the core of the cylindrical laser device. To cool the core which consists of a bundle of optical fibers as well as to cool the anode, cooling fluid flows through the space between the fiber bundle and the anode. This space is parallel to the axis of the fiber bundle. Spacing rings are present between the fiber bundle and the anode. The rings are said to force coolant into the interstitial spaces of the bundle. The Flyer patent must not require close packing of the fiber bundle, since coolant is permitted to travel through interstices in the FO rod structure from this external space.
The closer the fiber rods are packed, the greater the pressure necessary for the coolant to access the interstices in this manner. If the interstices are accessed, the cooling fluid pools there and only exits by returning to the space external to the bundle.
In accordance with the utility of the laser as a light source, a coherent beam exits the end of the core. The Flyer patent does not address the problem of cooling the face of fibers which receives light to be transmitted.
U.S. Pat. No. 5,315,683 to Miller, which is incorporated by reference herein, reports a ventilated fiber optic bushing which holds a bundle of optical fibers together. In order to have a cooling gas travel through the interstitial spaces of the fiber bundle, there are numerous specific requirements of the apparatus, including the light source, for providing this cooling air. The bundle of fibers is clamped within an elastomeric tube, in order to clamp the fibers into another tube which is rigid. The rigid tube holding the fibers is inserted into a light-emitting aperture of a light projector. The window of the light-emitting aperture of the light projector, which is the same size and shape as the fiber bundle, is disposed opposite the end of the fibers. A cooling fan is disposed within the light projector to draw external air through the interstitial spaces of the fiber bundle and around the window. The ends of the fibers proximate to the window have convex ends. The convex shape assists in having the air which passes through the interstitial spaces travel across the illuminated face of the fiber bundles for cooling.
For the fan disposed in the projector to draw air through the interstitial spaces, both the light projector and the bushing must be within a sheath to prevent the movement of extraneous air. The apparatus is designed exclusively for the use of a gas coolant.